If you do indeed have a Mac Pro according to Apple menu -> About This Mac, there is no reason why you can't go straight to 10.6.8:
Above that, read this tip:
It is cheaper than 10.5, and will give you more compatibility with more software that is current.
PowerMac G5, G4 867 Mhz or higher (1 Ghz is higher), Powerbook G4 867 Mhz or higher, iBook, iMac G5, iMac G4 1 Ghz higher, eMac 1 Ghz or higher
can all only go up as high as 10.5.8, but also lose compatibility with Classic. Some of those machines can dual boot Mac OS 9 from 2001 & Mac OS X 10.5 as this article says:
800 Mhz and less, including dual processors are stuck with 10.4.11.